Modern inorganic chemistry

THE ELEMENTS OF GROUPS IB AND MB 431

can, however, be attacked by certain substances, of which three may
be mentioned:

1. In the presence of air, it is attacked by potassium cyanide
   solution, to give the complex dicyanoaurate(l) ion, in which gold has
   an oxidation state + 1:
   4Au + 8CN~ + 2H 2 O + O 2 -> 4[Au(CN) 2 ]~ + 4OH"


2. It is dissolved by^4 aqua regia' (a mixture of concentrated
   hydrochloric and nitric acids). The product here is chlorauric(III)
   acid, HAuCl 4 ; in the complex chloraurate ion [AuCl 4 ] ~ gold is in
   oxidation state + 3, auric gold.*


3. It is dissolved by bromine trifluoride, to form finally gold(III)
   fluoride, AuF 3. This is a notable compound, for in it gold exhibits a
   simple valency of three, whereas in all other gold(III) compounds,
   gold is 4-coordinate, usually by complex formation (see below).

GOLD(I) COMPOUNDS

These all tend to disproportionate into gold and gold(III) com-
pounds, as already stated. Some of those which are insoluble in
water, for example gold(I) sulphide, Au 2 S, are fairly stable; others,
for example gold(I) oxide, Au 2 O, readily decompose even on gentle
heating. One of the most stable is gold(I) cyanide, AuCN, which is
formed when the ion, [Au(CN) 2 ]~, is allowed to react with hydro-
chloric acid. Gold(I)iodide, Aul, is also formed by the slow loss of
iodine from the gold(III) iodide, (AuI 3 )n. (The stabilities of gold(I)
cyanide and iodide may be compared with those of the correspond-
ing copper(I) salts.)
Gold(I) salts of oxo-acids are not known, but many complexes of
gold(I) have been discovered.

GOLD(III) COMPOUNDS

In the gold(III) halides (except the fluoride) there is evidence for the
formation of double molecules, Au 2 X 6 (cf. chlorides of iron(III) and
aluminium) so that the coordination is brought up to four, but with
a planar structure:
c

\ /

c

\ /

Cl

Au Au
/ ^ / \
Cl Cl Cl

• The ion can be regarded as (Cl~ -*AuCl 3 ). and coordination by the chloride ion
  brings the covalency from three (in AuQ 3 ) to four (in [AuCl 4 ]^), the oxidation
  state remaining as + 3.